Abstract
Rene Descartes gave his conception of what later became known as 'substance dualism', setting off a vigorous, ongoing debate about the mind-body relationship. The primary tenets of his view included:
(1) a 'substance' is 'a thing capable of existing independently';
(2) mind and body both are substances, but
(3) mind is thinking and nonextended, whereas
(4) body is non-thinking and extended;
(5) the body has the property of exclusionary occupation of space; and
(6) the human being is an interactive union of mind and body.

Descartes never provided a mechanism of mind-body interaction, but felt that mind and body could share some characteristics. The focus of the present work is not on interaction, but on the possible nature of the hypothesized non-material mental substance. Descartes made radical distinctions between mind and body, but it is also true that subsequently these distinctions have been distorted by materialists to consist of the following description:
the non-physical mind has neither mass nor energy, is not in any way localizable, does not exist in space, has nothing in common with physical things, and works independently of the brain. This is even more radical than Descartes' proposals, and ignores many of his specific points. If both body and mind fit the definition of substance, then they must share some properties in order to belong to that category.

Descartes was in no position to offer a solution to this problem, but modern physics (relativity and string theory) offers a resolution. Relativity provides a relationship between matter and energy: the energy (E) in a given amount of matter is the mass (m) multiplied by the speed of light (c) squared (E = mc2). This defines matter energetically. One can thus view matter as a form of condensed energy, with a certain energy density. One can imagine a series of substances (any form of condensed energy), differing in their energy densities. For example, matter could be a particular case of a general substance-energy relationship, where energy (E) is the amount of substance i (Si) multiplied by the speed of light to the power i (ci). For matter, i = 2 (substance S2).
The mental substance could be represented by substance S1. Thus the mental substance would be much less energydense than matter. S1 would display basic differences from matter in the way it is constructed from energy, with important property differences. For example, substances S1 and S2, being very different in energy density, could simultaneously occupy the same three-dimensional space.

String theory shows how two different substances could be constructed. Strings are tiny strands of energy, the basic building blocks of everything, comprising two types: open-ended (OE) and closed-loop (CL). OE strings are attached on each end to our spacetime (universe), while CL strings are free-floating and capable of leaving this spacetime. In addition, CL strings are much less energy-dense than OE strings. Thus, OE and CL strings can be construed as different substances, with some shared and some differing properties, promising candidates for Descartes' substance dualism.